A. Field of the Invention
The present invention relates generally to wireless communications systems and, more particularly, to a system and method for optimally selecting guard bands in a dual mode network, such as a Code Division Multiple Access (CDMA) system overlaid on an existing Advanced Mobile Phone System (AMPS).
B. Description of Related Art
AMPS is an analog system that permits communication by mobile units operating within an analog cell site. CDMA systems, on the other hand, are digital systems that permit communication by portable units operating within a CDMA footprint; i.e., a geographical area offering digital CDMA service. In a dual mode network, the CDMA system is overlaid on an analog AMPS.
Because both the AMPS and CDMA system operate concurrently in the dual mode network, some interference inherently occurs. Generally, there are four possible interference mechanisms between the AMPS and the CDMA system both operating in the 800-900 MHz frequency band (i.e., base station transmit frequency in the range of 869-894 MHz and receive frequency in the range of 824-849 MHz): (1) interference from AMPS sites to CDMA portable units; (2) interference from AMPS mobile units to CDMA sites; (3) interference from CDMA portable units to AMPS sites; and (4) interference from CDMA sites to AMPS mobile units.
Even an interfering AMPS site located far from the CDMA system might cause interference to CDMA portable units due to the AMPS site""s power transmissions, antenna height, or other operating conditions. Any AMPS site that interferes with the CDMA system requires xe2x80x9cclearingxe2x80x9d of analog channels from the frequency band being used by the CDMA system. Clearing is an expression commonly used to refer to the action of decommissioning that part of the spectrum from a cell site.
If the AMPS site does not cause excessive interference to any CDMA portable unit due to a very high path loss, for example, the path loss from a mobile unit within the AMPS site""s serving area may still cause interference to the CDMA system if the path loss between the AMPS mobile unit and the CDMA base station is very low. This may occur when an AMPS mobile unit transmits over water, from elevated highways or mountain roads, etc.
Conventional systems typically ignore such interference mechanisms. All of these interference mechanisms, however, must be considered in a proper analysis of guard zones and guard bands. A guard zone is a geographical area in which cell sites must have a portion of their spectrum cleared to reduce interference between the analog AMPS and the digital CDMA system. A guard band refers to the amount of spectrum, or frequency, that must be cleared at an AMPS site lying in the guard zone, when the digital and analog systems are overlaid.
Conventional systems typically implement a standard number of guard bands equal to 9 AMPS channels wide on either side of the CDMA carrier at each AMPS site. It had been thought that this number of guard bands was needed to preserve channel quality and minimize interference. The standard use of guard bands of 9 AMPS channels results, however, in an unnecessary loss of spectrum in the AMPS network.
As a result, a need exists for optimally selecting the number of guard bands to minimize the loss of spectrum in the AMPS network.
Systems and methods consistent with the present invention address this need by optimally selecting the number of guard bands to minimize the loss of spectrum in the AMPS while preserving channel quality and minimizing interference.
In accordance with the purpose of the invention as embodied and broadly described herein, a system consistent with the present invention identifies analog guard band channels for reclamation in a dual mode system having a digital network overlaid on an analog network. The digital network includes multiple digital cell sites. The analog network includes multiple analog cell sites. The system measures an amount of interference caused by the analog cell sites and an amount of interference caused to the digital cell sites. The system then compares the interference amounts to one or more thresholds and determines for which of the analog channels the interference amounts exceed the thresholds. The system selects the determined analog channels for reclamation.
In another implementation consistent with the present invention, a computer program product, stored on at least one memory device, identifies analog channels for reclamation in a dual mode system having a digital network overlaid on an analog network. The digital network include multiple digital sites, and the analog network includes multiple analog sites. The computer program product includes a forward link analysis module and a reverse link analysis module.
The forward link analysis module determines the first amount of interference caused by the analog sites to portable units operating in the digital sites, compares the first interference amount to a first threshold, and identifies analog channels for reclamation based on a result of the comparison. The reverse link analysis module determines the second amount of interference caused by mobile units operating in the analog sites to the digital sites, compares the second interference amount to a second threshold, and identifies analog channels for reclamation based on a result of the comparison.